1. Field of the Invention
This invention relates to an exposure control circuit for use in a photographic or cinematographic camera including an automatic exposure controlling element such as an electric shutter or an electrically controlled diaphragm, and more particularly to an exposure measuring circuit including a photodetector in one arm of a bridge circuit connected with an electric exposure controlling element in a camera.
2. Description of the Prior Art
It has heretofore been well known in the art to measure the illumination of an object to be photographed by use of an electric circuit including a bridge circuit having a photodetector in one arm thereof. The imbalance output of the bridge circuit including the photodetector which receives light from the object varies with the variation in the brightness of the object. In the bridge circuit which has three arms beside the arm including the photodetector, three factors of exposure can be set. The number of the factors of exposure which can be set in the circuit is desired to be as large as possible.
The factors of exposure to be set in the exposure control circuit beside the information of the brightness of the object to be photographed detected by the photodetector are, for instance, shutter speed or open angle of a rotary shutter (in case of a movie camera), film sensitivity, diaphragm and frame speed (in case of a movie camera). One of the arms of the bridge circuit usually includes a fixed resistor to fix the level of the bridge circuit and accordingly the level of the correct exposure which is desired to be obtained. Further, in addition to these factors, it is sometimes desired to take the back-light into consideration and put a factor concerned with the back-light in the bridge circuit.
The conventional exposure control circuit including a bridge circuit has been limited in the number of exposure factors to be set therein. Therefore, in one example of the conventional movie camera including such an exposure control circuit to control a diaphragm, one arm is used to determine the level of the bridge circuit, a second arm includes a photodetector and used to measure the brightness of the object to be photographed, a third arm is used to set the film sensitivity, and the fourth arm is used to set either the open angle of the rotary shutter or the frame speed. In the fourth arm, a mechanical switch over means is provided to select the open angle of the rotary shutter or the frame speed. If the back-light control is desired to be set in the circuit in the conventional exposure control circuit, a mechanical switch over means must be provided in the arm used for the film sensitivity or in the arm used for the level control. The above described conventional exposure control circuit which requires a mechanical switch over means in an arm of the bridge circuit suffers from the complexity in construction and operation.